There is more to good vision than 20/20Your VisionMitchell Shultz, M.D.

There's more to good vision than "20/20"

If you're reading this, you're probably familiar with the vision chart that hangs in every optometrist's or ophthalmologist's examining room. The one with a big "E" at the top. It's officially known as a Snellen chart and it's been the basis for measuring what's known as visual acuity since the late 1800s. Think of visual acuity as a quantitative measure of your vision, establishing where your vision is placed on a numeric scale. There are also qualitative measures of your vision, such as your ability to perceive subtle contrast changes, especially in low-light situations. Taken together, these quantitative and qualitative measures determine the overall performance of your vision.

Visual acuity: your vision, by the numbers

If your visual acuity is determined to be 20/20, you see at 20 feet what a person with normal vision sees at 20 feet. If you have 20/40 vision, you see at 20 feet what a person with 20/20 vision would see at 40 feet. And, if you're one of the lucky ones (e.g., 20/15), you can see at 20 feet what others would have to move closer (15 feet) to see. Your visual acuity is an important measure of your vision. Many common problems can be identified by this measurement method, including the ones you've most likely heard of before " like nearsightedness and farsightedness.

Familiar vision problems

Over half the population of the United States experiences one or more common visual acuity problems. Generally, people with visual acuity problems are nearsighted, farsighted, and/or have astigmatism. Collectively, these conditions are medically referred to as lower order aberrations. Lower order aberrations are usually corrected by wearing glasses, contact lenses or by laser vision correction.

Understanding your prescription

If you've seen the vision prescription your optometrist or ophthalmologist prepares following an eye exam, you've probably seen a series of positive or negative numbers, one for each eye. These values are known as diopters, and they typically measure the refractive power of your eyes on a scale of -14 to +14. A person with 20/20 vision will most likely have a reading of zero diopters. A person with myopia or nearsightedness would have a negative diopter value; a person diagnosed with hyperopia or farsightedness would have a positive diopter value. Regardless of whether the number is negative or positive, a higher number indicates a higher refractive error. Your prescription specifies just how much vision correction is required to correct your particular vision problems.

To understand these refractive errors, compare the information and illustrations below of an eye with good visual acuity with a myopic (nearsighted), hyperopic (farsighted) or astigmatic eye.

Good visual acuity

What you've probably always heard is true: the human eye does work like a camera. The light and images we see pass through the cornea at the front of the eye. The light and images then go through the lens inside the eye, and, finally, focus directly onto the retina, at the back of the eye. The retina sends the "signals" to our brain, which registers them.

Poor visual acuity is primarily caused by refractive errors. These errors occur when the cornea is shaped in such a way that the images we see do not focus directly on the retina.

Nearsightedness (Myopia)

When you're nearsighted or myopic, images in the distance will seem blurry. Your eyes may be longer than normal or the cornea may be too curved, so images focus in front of the retina.

Farsightedness (Hyperopia)

When you are farsighted or hyperopic, images that are near (the words on a page, for example) appear to be more blurry than images in the distance. Your eyes may be too short, or your cornea too flat, so images focus behind the retina.

Astigmatism

Astigmatism results in a blurring of all images, whether near or far. Here, images focus on more than one point in front of, or behind the retina. The result is that all images, whether near or far, may be blurry. In mixed astigmatism, symptoms of myopia or hyperopia are combined, resulting in the overall inability to see images clearly.

When making your decision about laser eye surgery, realize that many of the methods of correction you may have already heard about, such as glasses, contacts or traditional LASIK, are capable of correcting these lower order aberrations.

Other common vision problems.

Presbyopia, another familiar vision problem, is different from any of the problems discussed above. A physiological weakening of vision due to the natural aging process, presbyopia is the reason many people require reading glasses from the time they reach middle age. Since presbyopia reflects a problem with the eye's lens, and not its cornea, it cannot be corrected by LASIK or other types of corneal vision surgery. However, new exciting technologies are available to surgically correct Presbyopia.

Beyond the numbers: your vision's quality

There are also qualitative factors affecting your vision. A number of considerations " like contrast sensitivity " also contribute to your vision's performance. Contrast sensitivity is a measure of the ability to discern subtle changes in a visual image. Many routine visual tasks " recognizing a face, for example " rely more upon contrast sensitivity than visual acuity. Problems believed to be associated with poor visual quality are normally most apparent in low-light situations.

Understanding your vision requires taking both the quantitative and qualitative measures into consideration. This is because it's possible to possess very good visual acuity yet have poor visual quality, and vice versa. Dr. Shultz will determine which procedure is best for your eyes.